As she revealed in a new interview, it was over her song "Ooh My Love," which, in its original form, was unintentionally lifted from Petty's Let Me Up (I've Had Enough) track "Runaway Trains."

"I stole that from Tom Petty — accidentally!" Nicks told Yahoo! Entertainment. "I picked up the wrong cassette at Tom’s one night, a tape of Mike Campbell’s instrumental demos. Tom would get them first, and then the ones he didn’t want, Mike sent them to me. I accidentally arrived home one night with a cassette — I thought it was mine, but it was Tom’s. It just said, '24 Demos From Mike Campbell.' It had the song that inspired 'Ooh My Love,' which became 'Runaway Train[s]' for Tom."

Listen to Stevie Nicks' 'Ooh My Love'

Listen to Tom Petty and the Heartbreakers' 'Runaway Train'

Nicks said she took the song to Fleetwood Mac "and sang my lyrics over it. We started to record. I loved it so much, I called Tom and said, 'Listen to this!' What an idiot, right? Let’s play him the song you stole over the phone! Tom just starts screaming at me on the other end of the phone. I’m realizing, 'How stupid are you, Stevie?' So I had to go in the next day and tell Fleetwood Mac, 'Guess what, we can’t do this song.' 'Why can’t we do it?' 'Because I stole it from Tom Petty, and I’m absolutely a total criminal and a thief.'"

Nicks erased the recording, but she held on to the lyrics. "Then way later, years down the road, I sat down at the piano and tried to recall it," she said. "I wrote 'Oooh My Love' on the piano: 'In the shadow of the castle walls … ' Of course, I don’t know near as many chords as Mike Campbell does. All I remembered was that distant enchanted melody. ... Me and Tom and Mike Campbell, we’re like quintessentially three parts of one person."